Here is an interesting discussion concerning the 5W-20 and 5w30 debate:
"Thinner motor oils such as 5W-20 or even 0W-20 are becoming more popular these days and are even specified by some OEM's (FORD & HONDA) on new 2001 cars.
Although these oils are promoted as "energy conserving" they generally trade a gain of less than 0.1 MPG in Corporate Average Fuel Economy (CAFE) for shorter useful engine life.
FORD which has previously designed cars to have 10 year or 150,000 miles life has reduced the mileage life expectation to "beyond 100,000 miles" on vehicles that are operated on SAE 5W-20 Motor Oil.
HONDA only claims "useful life" as 7-years or 70,000 miles in EPA certifications for their CIVIC which uses SAE 5W-20 Motor Oil, while the previous model that utilized SAE 5w30 Motor Oil was certified for 10 year or 100,000 mile durability.
Since both HONDA and FORD Warranty their NEW cars for ONLY 3-years or 36,000-miles the reduction in engine life expectancy is not a factor.
By contrast Mercedes-Benz recommends use of ONLY Synthetic Motor Oil that is at least SAE 5W-40! This is a recent increase in recommended viscosity from SAE 5w30. Apparently customer research indicated that engine longevity is more important to typical MB customer than fuel economy."
http://www.synlube.com/viscosit.htm#SAE J300 Viscosity Classification (Current)